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1. Field
The present system relates to the field of variable, passive, electronic components.
2. Electronic Art
Throughout the history of electronics, the inherent characteristics and consequences associated with the presence of fixed value passive components have impacted circuit performance. The characteristics include component tolerances, tolerance build-up, the relatively high cost of using high precision components, added circuitry required to provide precision control or precisely set component values or performance characteristics, and changes in component value or performance produced by component aging, operating history, and changes in environmental conditions.
FIG. 1 illustrates an idealized solution in the form of variable resistor R100, variable capacitor C100 and variable inductor L100. These components would be comparable in nature to the present passive devices they would replace, but whose value could be adjusted to provide nearly nominal performance over widely varying operating conditions. Variability would either be part of the manufacturing processes, or dynamically available during actual operation, as determined on a case-by-case basis. One useful application illustrated is directed toward providing a matched, complex termination (Z100) for a transmission line structure (TL100).
As a result of the long-recognized benefits associated with use of variable passive components, techniques have been made developed to provide this capability using the present art. The effectiveness of the techniques is generally limited to specific types of components over a limited range of operating conditions, such as frequency or power level. One example of a digitally controlled, variable resistor of the present art is shown in FIG. 2A. A switched resistor network is used with control FETs such that one or more FETs “on” places resistors in parallel, reducing the effective resistive value of the combination. Resistor networks are more frequently implemented as series structures with one or more FETs used to short out unwanted resistors in the chain. For resistors, value selection controls are far simpler with series connection. Switched capacitor structures are typically configured as parallel networks, again simplifying value selection controls.
Other implementations of variable value components generally realize the function but can have significant impact on the circuit or limited performance. FIG. 2B illustrates the use of the channel resistance of a FET as a variable resistor (note resistor R206 is variable implying some form of gate voltage control). Among the undesirable characteristics of this approach are the large number of unused components and the characteristics of the FET switches. When a value has been selected, there are typically many unused components. This is typically not a major issue for an integrated resistor network but switched capacitor networks can require discrete capacitors to realize large values, adding cost and using circuit board area that could be used for functioning circuitry.
The switch FETs also can present significant problems. If kept small to minimize chip area utilization, they tend to have significant resistance that is aggravated by their rapid resistance increase with temperature (a sensitivity experienced to an even greater degree by the circuit for FIG. 2B). If the FETs are enlarged to reduce resistance, they consume greater area. Either way, there is an incentive to minimize the number of switched devices (4-bit is most common with 8-bit the maximum generally encountered), thereby limiting resolution of the component value.
FIG. 2C illustrates a varactor diode, which provides the function of a controlled variable capacitor. Implementation requires a DC control voltage and DC isolation. Use is generally limited to RF applications where the small diode capacitance under reverse junction bias yields appropriate values for circuit operation.
Inductors are particularly difficult components in which to implement variability because they frequently are wound around a magnetic core material. Modification of the geometric relationship between the core material and the winding provides inductance adjustment capacity. However, the adjustment capacity is typically a mechanical setting with real time adjustment being largely impractical, particularly at high frequencies.
A significant issue associated with realization of component value variability is energy storage, which is a function of the component value (whether primary or parasitic in nature). Alteration of a component value typically involves movement of energy into or out of the storage mechanisms associated with the specific type of passive component, thereby significantly limiting the speed of circuit operation. For many applications, it would be highly beneficial to be able to change apparent component value without requiring significant energy transfer.
It would therefore be highly desirable to implement low cost, high-resolution value variation for common passive components.